1. Field of the Invention
This invention relates to microphone biasing circuitry and methods for reducing or removing unwanted bias on a microphone contact of a connector (for connecting to an external microphone) as a removable peripheral apparatus is removed.
2. Description of the Related Art
Many contemporary electronic devices have the facility to connect with peripheral audio devices. For instance, mobile telephones, tablets, laptop computers and the like are examples of electronic devices that are operable with peripheral audio devices such as a headset, for example, that is external to and distinct from the electronic device. Headsets typically comprise mono or stereo speakers for audio playback and a microphone for voice communication.
Such external peripheral audio devices are often connected via a mating connector such as a plug and socket arrangement. For instance, many headsets have a 3.5 mm jack plug for connection to a suitable jack socket on the host electronic device. A well-known arrangement for a jack plug and its associated socket is TRRS (Tip-Ring-Ring-Sleeve), which has four contacts for left audio, right audio, microphone, and ground. In one known arrangement, the tip (T) and first ring (R1) are used for left (L) and right (R) audio, for example left and right loudspeakers, with the second ring (R2) and sleeve (S) used for the microphone (M) and ground (G) respectively. It will be appreciated that different arrangements for the left and right audio, microphone, and ground contacts are also possible.
In use a microphone bias is supplied by the host device to the microphone (M) contact of the socket so that when a plug (which is connected to a headset with an external microphone) is plugged into the compatible socket the external microphone is suitably biased and is ready for use.
Using the well-known arrangement described above as an example for illustrating the problem, as the plug is removed from a socket some of the contacts (T, R1, R2, S) on the plug will be moved past and make contact with other various and different contacts (T, R1, R2, S) of the socket. This can lead to undesired effects if there is still a voltage present on a socket contact. For example, when the plug is removed from the socket the tip contact T and first ring contact R1 of the plug, which are connected to the speakers (L, R) of the peripheral apparatus, will make contact with the socket contact R2 contact used to provide bias for the external microphone. If there is still a bias present at this socket contact R2 then this unwanted bias causes the speakers to operate in an undesirable fashion, e.g. to output an audible artefact such as a pop or click sound.
FIGS. 1a and 1b illustrate this effect. FIG. 1a shows a jack plug 110 fully inserted into a jack socket. The jack socket comprises contact pins 113, 115, 117, 119 serving as contacts T, R1, R2 and S of the socket respectively. (The remainder of the socket mechanical structure is omitted for clarity). The jack plug has four contact areas, 112, 114, 116 and 118 serving as contacts T, R1, R2, S of the plug respectively. In this example these plug contacts may be connected to a first (e.g. left) speaker L, a second (e.g. right) speaker R, microphone M, and common ground return path G respectively as shown in FIG. 1c which illustrates the connections in the removable peripheral apparatus. However the configuration of the socket contacts and the plug contacts may differ.
FIG. 1a shows the jack plug as being fully inserted into the socket with each plug contact making contact with the correspondingly correct socket contact (and only that socket contact). Circuitry may be connected to the four contacts of the socket consistent with this connection, for instance microphone bias and amplifier circuitry connected to the socket R2 contact, a ground connection to the socket S contact, and right- and left-channel speaker driver amplifiers to the socket R1 and T contacts.
The socket may also comprise some circuitry, for instance a normally-open switch 111 which is closed when the jack plug is fully inserted, to allow generation of a jack-insert detection signal, for instance by means of an external pull-up resistor connected to one pole of the switch, to indicate whether there is a plug fully inserted into the socket or not.
FIG. 1b shows the plug having been partially removed. In this state the plug sleeve (plug S contact 118) connected to the common ground return G has been completely disconnected from the socket. The plug R2 contact 116, connected to the microphone, is now in contact with the socket S contact 119, which is grounded. The plug R1 contact 114, connected to the second speaker (R), is now in contact with the socket R2 contact 117, itself connected to the microphone bias circuitry. Therefore there will be a bias on the socket R2 contact 117, and such bias will cause current to flow from this contact, through the second speaker R and microphone M to ground via socket S contact 119.
Furthermore when the jack plug is pulled out further the plug T contact 112 connected to first speaker L will (at least briefly) make contact with socket R2 contact 117 connected to the microphone bias, and the plug R1 contact 114 connected to second speaker R will make contact with socket R2 contact 117 connected to the microphone bias. Thus it can be seen that removing the plug will result in the microphone bias being temporarily applied to both the left (L) and right (R) audio speaker contacts (T, S1) on the plug which will result in current passing through both speakers via their common connection, which may result in an unwanted sound, i.e. an unwanted audio artefact.
In both FIGS. 1a and 1b is illustrated a typical configuration of microphone biasing circuitry. In this example microphone biasing circuitry 102 such as a buffer amplifier outputs, via resistor 121, a bias for the external microphone. The microphone output, typically a JFET source follower with gate coupled to a capacitive electret (or MEMS) acoustic transducer, superimposes an AC signal on the d.c. bias provided. The input circuitry 103 buffers and amplifies the AC signal produced by the microphone in use. The DC blocking capacitor 124 allows the audio signal from the microphone to pass though to the input amplifier, but blocks the quiescent d.c. level of the microphone output from the d.c. bias of the input amplifier to allow the latter to be set independently at a convenient level, typically half the supply voltage.
Shown in FIG. 2 is another known configuration for biasing a microphone of a removable peripheral apparatus. For simplicity, the connector arrangement, e.g. plug and socket is omitted. The biasing circuitry will, in use, be connected to a suitable contact on a device connector, such as a microphone socket contact, i.e. a socket contact to which a microphone is anticipated to be connected via a corresponding microphone jack plug contact. These contacts may correspond to the R2 contacts 117 and 116 in the example of FIGS. 1a and 1b, but as noted could be other contacts in other examples.
The microphone biasing circuitry 102, generating the bias for the external microphone, connects to the microphone (M) socket contact 117 (R2) via two series connected resistors 222 and 223. Between the two resistors 222, 223 there is a capacitor node to which the first plate of a capacitor 225 connects. The other plate of the capacitor 225 connects to a reference voltage node, in this case a ground node. This capacitor 225 serves as a low pass filter to remove any noise which could be picked up on the microphone bias relative to this ground node and would otherwise appear as a spurious component of the input signal. In mobile telephones for example, a significant source of noise is due to the GSM RF power amplifiers, which creates TDMA noise (time division multiple access noise) due to supply current modulation appearing on the ground return paths for example at a frequency of 217 Hz. In order to prevent this noise being picked up by the microphone circuitry the capacitor 225 has to be large enough to significantly attenuate signals of a frequency of 217 Hz. This means that during operation the capacitor will store an appropriate amount of charge which will eventually have to be discharged at some point in time.
In order to at least reduce the unwanted bias at the microphone socket contact the microphone biasing circuitry 102 may be disabled, for example by removing supply power to the amplifier, as the jack plug is removed from the socket. However, the large capacitor 225 will take time to discharge, and meanwhile the charge present on the capacitor 225 will still create an unwanted bias at the microphone socket contact. This unwanted bias from capacitor 225 will then operate the first and second speakers connected to the jack plug in an undesired fashion as described above.